Carbon dioxide gas lasers have been used in materials processing for quite some time. Carbon dioxide lasers are favored since they tend to generate relatively high powers for lower costs than other laser systems.
For certain applications such as engraving, data encoding, ceramic scribing, marking and paper perforation, pulsed carbon dioxide lasers with high repetition rates are desired. Sealed gas lasers available in the prior art are capable of generating usable outputs of up to 10Khz. Above that rate, the rise and fall times of the pulses tends to approach the length of the pulses and therefore the laser will not fully turn off between pulses. The extent which the laser will turn off between pulses can be defined by the percentage or depth of modulation. Under this definition, if there is no laser output between pulses, then the depth of modulation is 100 percent. On the other hand, if the laser generates an output in the period between excitation pulses, the depth of modulation is less than 100 percent and can be calculated based on the following formula: ##EQU1## As can be seen from equation (1), if the output power between energizing pulses drops to only one half of the maximum output power, the depth of modulation is 50 percent.
In the applications mentioned above, it is necessary to achieve modulation levels well over 50 percent and close to 90 percent is desired. As can be appreciated, when the beam is passing over a portion of material which should not be treated, the power should be off. If a significant portion of beam power is present, unwanted scoring, marking or partial perforations will occur in the work piece.
In order to achieve higher repetition rates, various material processing systems have utilized flowing gas lasers. In these lasers, the lasing gas is continuously recirculated. This approach permits the generation higher repetition rates with a good depth of modulation because the gas molecules storing the excitation energy are swept out of the cavity, thereby shortening the pulses. Unfortunately, flowing gas lasers are more complicated and require blowers and large reservoirs of carbon dioxide gas. Due to this added complexity and requirement for extra materials, the flowing gas lasers can cost twice as much as sealed gas lasers.
Due to the drawbacks of flowing gas lasers, significant effort has been made to increase the repetition rates of sealed carbon dioxide lasers while maintaining the desired modulation levels. For example, it is known that if the gas pressure is increased, the pulse widths can be somewhat shortened.
Another approach to shortening pulses is to vary the gas mixture. In a typical carbon dioxide laser, the gas mixture includes helium and nitrogen. The helium acts as a buffer gas while the nitrogen cooperates directly in the energy transfer. More specifically, nitrogen has a band gap energy similar to the energy gap between the ground state and the lasing state of carbon dioxide. Since there are no intermediate states in nitrogen, the atom acts as an energy storage device which can transfer that energy to the carbon dioxide molecules through collisions.
Nitrogen is a very important constituent of the gas mixture when longer, higher power pulses and greater efficiency is desired. However, in the applications discussed above, where short pulses are desired and relatively low power pulses are acceptable, the level of nitrogen in the gas mixture can be substantially lowered or even eliminated. In this manner, the fall time of the pulses can be significantly reduced allowing the repetition rates to be increased.
While variations in pressure and gas mixture have provided some improvements, the sealed gas lasers found in the prior art still cannot compete with the repetition rates attainable with the flowing gas lasers. Accordingly, it would be desirable to design a sealed gas laser which can generate a pulsed output having a repetition rate and modulation characteristics which would compete with the flowing gas lasers.
Therefore, it is an object of the subject invention to provide a sealed gas laser capable of generating a pulsed output having a high repetition rate.
It is a further object of the subject invention to provide a sealed gas laser which can generate a high repetition rate with a high level of modulation.
It is another object of the subject invention to provide a sealed carbon dioxide laser which can generate a pulsed output of up to 30Khz with a depth of modulation in excess of ninety percent.
It is still a further object of the subject invention to provide a sealed laser which utilizes an isotope of carbon dioxide as the lasing gas.
It is still another object of the subject invention to provide a sealed isotopic carbon dioxide laser which generates pulses having shorter rise and fall times.
It is still a further object of the subject invention to provide a sealed isotopic carbon dioxide laser wherein the pulse widths are much shorter than with standard carbon dioxide.
It is still another object of the subject invention to provide a laser system for materials processing which utilizes an isotopic carbon dioxide laser.
It is still a further object of the subject invention to provide a laser system utilizing an isotopic carbon dioxide laser having an output of 11.1 microns which will not be absorbed by a carbon dioxide assist gas.